The US 2010/0310423 A1 discloses a biosensor cartridge having a well with a sensor surface at which sample particles can be detected by Frustrated Total Internal Reflection (FTIR) of a light beam. In an alternative embodiment, the sensor surface may have a jagged surface structure by which light is directed for a short distance through the sample. With the described biosensor cartridge, a sample can be examined in a region close to the sensor surface.